1. Field of the Invention
This invention relates to a recording apparatus which can solve various problems arising due to the existence of heat, and more particularly, it relates to a recording apparatus in which the heat must be considered, for example, such as an ink jet printer having a thermal fixing means.
2. Related Background Art
To date, ink jet printers have been advantageous in that there was no need to use a specific fixing means for recording paper.
However, recently, ink jet heads having rapid or prompt response features and microprocessors capable of performing high speed operations have been proposed, and high-density multi-orifice ink jet heads and longer ink jet heads have been developed in accordance with the progress of high accuracy and/ or super precision working techniques.
Consequently, the recording speed of the ink jet printer has rapidly been increased, and thus, some problems which were not regarded as important up to date have arisen.
In the ink jet printing system, liquid or molten ink drops are jetted onto a recording paper i.e., recording medium. The fixing of the ink on the recording medium is effected by the ink penetrating into the recording medium or by vaporizing the solvent in the ink (by means of a vaporizing process) to fix the ink pigment on the recording medium, thereby forming an image on the recording medium.
More specifically, the ink attached to the recording medium (normally, a plain paper) is fixed on the recording medium by the ink penetrating into the medium by capillarity and/or chemical bonding force, or by leaving the ink pigment or ink dyes on the recording medium by evaporating the solvent in the ink liquid.
However, in these cases, the fixing speed depends upon not only the configuration and physical features of the recording medium but also upon the surrounding atmosphere. Further, the time when the ink is naturally or unartificially fixed on the recording medium cannot be reduced beyond a certain limit due to the physical properties of the ink and medium.
Therefore, in recent high speed printing operations, there has arisen the problem that the recording medium was ejected before the fixing of the ink was completed. In such a case, the printer or an operator's finger is smudged with unfixed ink, or the back of the next recording paper is smudged with unfixed ink on the previous recording medium, or an image of high quality cannot be obtained due to the flow of unfixed ink on the recording medium.
In order to prevent the worsening of the recording quality due to such imperfect fixing, for example, a special recording medium (such as coated paper and the like) having an absorbable coating layer thereon has been used to shorten or decrease the fixing time. In this case, however, the important advantage that plain paper can be used in the ink jet printing system is lost, and a new problem that the use of the special recording paper prevents the wide development of ink jet printing system arises.
On the other hand, a technique in which the fixing means includes a heating means for heating the recording medium so as to decrease the fixing time has also been proposed.
For example, West German Patent Laid-Open Patent No. 2,717,119 discloses a technique in which the thermal fixing is effected by heating a recorded recording sheet by means of a flat heater. With this technique, the fixing efficiencY is improved; however, this proposed technique is still insufficient. That is to say, in this proposed technique, the heated surface (i.e., the back) of the recording sheet tends to shrink, thereby curling the recording sheet. Further, since the front surface of the recording sheet is supplied with ink liquid by means of the ink jet head, the front surface of the recording sheet tends to expand due to moisture of the ink, thus increasing the curl of the recording sheet.
If the recording sheet is so curled, the efficiency of heat transmission to the recording sheet is considerably decreased; thus, in order to obtain the desired fixing, the caloric value or heating value of the heater must be increased. However, the increased caloric value not only increases the consumption of energy, but also tends to dry the ink jet head, thus jamming orifices of the ink jet head. Nevertheless, there was room to improve the efficiency of the heating and/or to obtain a compact construction of the ink jet printer.
Further, in recording apparatuses such as ink jet printers and wire dot printers, the distance between a recording head and a recording sheet (referred to as "head gap" hereinafter) must be relatively severely controlled. If the head gap is not maintained properly, in case of the wire dot printer, since impact force cannot be maintained uniformly in the printing cycles, there arises a difference in density between the printed images, and in case of the ink jet printer, deflected ink dots are recorded on the recording sheet. Further, if the recording sheet contacts the printing head, the surface to be recorded (of the recording sheet) will be smudged.
Therefore, normally, in a recording apparatus, as shown in FIG. 10, a recording sheet 103 wound around a platen 101 and being fed to a recording position by means of the platen and pinch rollers 102 is pressed against the platen 101 just ahead of a recording head 104 by means of a sheet holding plate 105. More particularly, by pressing the recording sheet 103 against the platen 101 tangential thereto with uniform force by means of the sheet holding plate 105 the recording sheet 103 is maintained parallel to a face of the recording head 104, thus ensuring a proper head gap.
Further, in the recent printer market, there has been a tendency that special use printers have been modified to function as general use printers (that is, a tendency that the printer is popularized); for example, a printer which can print or record characters on any recording sheet has been requested. In this regard, since the wire dot printer is an impact-type printer, it can easily record the characters or images even on a plain paper; however, in case of the ink jet printer, since the ink is used as a printing medium, there was a problem of the fixing of ink on the recording sheet. Thus, it has been considered that in the ink jet printer plain paper could not be used since it was difficult to fix the ink on recording sheets other than special sheets.
However, recently, various methods and systems for fixing the ink on the recording sheet have been proposed. Among them, as a fixing apparatus having a relatively simple and compact construction, an evaporating fixing apparatus which includes a heater arranged inside a platen roller or arranged in a paper path other than that associated with rollers and wherein the fixing is effected by heating a recorded surface of the recording sheet by means of the heater to evaporate the moisture in the ink has been put to practical use. For example, in an apparatus shown in FIG. 11, a heater 106 is arranged inside a platen 101 and a recording sheet 103 fed by pinch rollers 102 is heated adequately before it reaches a recording head 104. Alternatively, in an apparatus shown in FIG. 12, a heater 108 is arranged behind a heating plate 107 which also acts as a flat platen. A recording sheet 103 fed by pinch rollers 102 is pressed against the platen roller 101 by a sheet holding plate 105 and then is fed to the heating plate 107, where the ink drops jetted onto the recording paper are vaporized to fix the ink dyes on the sheet
However, in the above-mentioned printers having a heater, it is, of course, necessary to provide a sheet holding plate. In these printers, since the heat is transmitted from the heated recording sheet to the sheet holding plate, the sheet holding plate is deflected, as shown in FIG. 13, due to the difference in thermal strain derived from uneven temperature distribution on the sheet holding plate. More particularly, in FIG. 13 showing a printer having a carriage 110 and a heater (not shown) incorporated into a platen 101, a recording sheet 103 introduced into the printer along a sheet guide 109 is heated by the heater in the platen 101. Further, the recording sheet 103 is pressed against the platen 101 by means of a sheet holding plate 105. In this connection, since an edge 105A of the sheet holding plate 105 is in contact with the heated recording sheet 103, the edge portion 105A of the sheet holding plate is locally heated; whereas the remaining portion of the sheet holding plate is not so heated as the edge portion, since the heat transmitted to said remaining portion is dispersed through a frame 111 by which the sheet holding plate 105 is supported. Consequently, corrugated deflection as shown in FIG. 13 is created on the edge portion 105A of the sheet holding plate.
In such circumstances, the recording sheet 103 cannot uniformly be pressed against the platen 101 due to the deflection of the sheet holding plate, with the result that the recording sheet floats partly above the platen, thereby causing disorder of the recorded image, oblique movement of the recording sheet and the like; further, in this case, if the recording sheet 103 contacts the recording head, the sheet will be smudged with ink. Of course, such condition has an undesirable appearance and causes functional defects.